A nuclear reactor generally has a vessel within which a reactor core is positioned and primary coolant flows. Commercial reactors commonly use water as the primary coolant, although reactors are operative where liquid metal sodium is used as the primary coolant. The reactor core itself has a large network of elongated passageways and nuclear fuel in the form of elongated fuel pins is located within certain of the respective passageways according to a specific cross sectional array. Neutron absorbing elements are likewise formed as elongated members and are positioned in certain of the other passageways, again according to a specific cross sectional array. The precisely defined matrix between the various fuel pins and neutrons absorbers and the controlled extent of lateral overlap or relative degree of presence of each within the matrix determine the extent and rate of the nuclear fission in the reactor core. Control rods extending into the reactor vessel are used to move the fuel pins and/or the neutron absorbers axially relative to one another to achieve the intended control overlap.
The primary coolant is forced through the reactor core passageways and directed then by appropriate piping or line means to heat exchangers, wherein a different or secondary coolant removes the heat from the primary coolant. The primary coolant in turn is circulated back to the reactor vessel for passage again through the reactor core passageways. The secondary coolant, generally pressurized water, is vaporized and this high pressure steam is expanded through steam driven turbines which power generator equipment that produces useful electrical energy.
The fuel pins might commonly be formed of a uranium oxide or plutonium oxide, cladded with a durable material such as zirconium or stainless steel; and the reactor core itself might be fabricated of stainless steel. The reactor vessel immediately below the reactor core typically has a gridwork therein for directing the primary coolant upwardly from an underlying plenum space into and through the passageways of the reactor core. Pumps of course normally would force the coolant into the plenum.
In the event the reactor should malfunction and begin to overheat, the possibility exists that the fuel pins or the cladding on the fuel pins might fracture and/or melt and interact with the coolant to form small particles of nuclear debris which would drop or fall then through the coolant to the underlying gridwork or reactor vessel. The nuclear debris could reach significant heights, as measured in terms of centimeters, and represent the problem within which this invention is directed.
The cause of overheating or malfunction could be reduced circulation of the coolant, even though the entire plenum may yet be, and very frequently will be, filled with the liquid coolant. The nuclear debris will be highly radioactive and thus will generate heat having a high heat flux. The coolant directly overlying the debris will cool at least the exterior particles of the debris, and convective circulation within the plenum space will normally be sufficient to keep the coolant at the debris surface in the liquid phase. Inasmuch as the coolant cannot rapidly get to all of the interior particles of the debris because of the torturous or long flow paths the coolant must take to reach these particles, the coolant frequently can be vaporized only slightly below the debris surface. The vaporized coolant normally moves upwardly through the overlying particles in the pile, and thus further hinders liquid coolant penetration into the debris interior for cooling the particles thereat. This depletion of coolant toward the interior of the debris allows localized "pile dryout", where temperatures up to possibly even 3000.degree. C. can be generated. These dryout temperatures generally exceed the design limits of the structural materials forming the reactor vessel and cavity, which thus could be structurally damaged. Destruction of the reactor vessel and the containment vessel, could lead to the escape of the coolant and the nuclear debris from such confinement, which in turn could allow the release of detrimental radioactivity to the environment and its resultant threat to public safety. Repairs of the reactor would, of course, also be most costly.